Project Summary Loss of neurons in the hilus of the dentate gyrus is one of the most consistent morphological findings in humans with temporal lobe epilepsy and related models of acquired epilepsy. Two major groups of hilar neurons are frequently damaged, GABAergic somatostatin interneurons and glutamatergic mossy cells. Despite years of interest in hilar neurons, important questions persist. Why are the neurons so vulnerable to excitotoxic damage? How could they be protected? What are the functional effects of both loss and preservation of subgroups of these neurons? A set of new technologies, reagents and mice will now allow probing these questions in exciting new ways. The proposed studies will use mice with Cre-recombinase expression in these neurons to selectively manipulate them through Cre-activated viral gene expression. A combination of light and electron microscopic immunohistochemical methods will be used to evaluate the changes, and functional correlates will be determined with electrophysiological methods. The broad goals are to determine if increasing tonic GABAergic inhibition in these neurons could be neuroprotective and, in a separate set of studies, to map the functional circuits that are activated following manipulation of either mossy cells or somatostatin neurons in normal and seizure-prone animals in vivo. Specific Aim 1 will test the hypothesis that both groups of hilar neurons lack substantial expression of the subunit of the GABAA receptor (GABAAR) and have low levels of tonic inhibition. Specific Aim 2 will test the hypothesis that expressing an exogenous GABAAR subunit, which is normally involved in tonic inhibition in the cerebellum, will lead to the formation of functional GABAA receptors and increase tonic inhibition in both groups of hilar neurons. Specific Aim 3 will test the hypothesis that increasing tonic inhibition in hilar neurons will protect them from damage following status epilepticus. Specific Aim 4 will examine the functional circuitry of hilar neurons in vivo by using optogenetic methods to manipulate the neurons selectively and then identifying the activated neurons by Fos labeling. The patterns of activation in normal and seizure-prone animals will be compared to test the hypotheses that stimulating remaining mossy cells or silencing remaining somatostatin interneurons in vivo will lead to increased granule cell activation in the epileptic mice. By allowing selective manipulation of hilar neurons in the intact brain, these studies will provide unique views of their function within normal and altered neuronal circuits.